1. Field of the Invention
This invention relates to a method to reduce static and kinetic friction between slidable surfaces, and to articles of low friction prepared thereby.
2. Description of the Prior Art
Certain devices require slow and controlled initiation and maintenance of sliding movement of one surface over another surface. It is well known that two stationary surfaces having a sliding relationsip often exhibit sufficient resistance to initiation of movement that gradually increased pressure applied to one of the surfaces does not cause movement until a threshold pressure is reached at which point a sudden sliding separation of the surfaces takes place. This sudden separation of stationary surfaces into a sliding relationship is herein referred to as breakout.
Breakout is particularly troublesome in liquid dispensing devices, such as syringes, used to deliver small, accurately measured quantities of a liquid by smooth incremental line to line advancement of one surface over a graduated second surface. The problem is also encountered in devices using stopcocks, such as burets, pipets, addition funnels and the like where careful dropwise control of flow is desired.
The problem of breakout is related to friction. Friction is generally defined as the resisting force that arises when a surface of one substance slides, or tends to slide, over an adjoining surface of itself or another substance. Between surfaces of solids in contact, there may be two kinds of friction: (1) the resistance opposing the force required to start to move one surface over another, conventionally known as static friction, and (2) the resistance opposing the force required to move one surface over another at a variable, fixed, or predetermined speed, conventionally known as kinetic friction.
The force required to overcome static friction and induce breakout is referred to as the breakout force, and the force required to maintain steady slide of one surface over another after breakout is referred to as the sustaining force. Two main factors contribute to static friction and thus to the breakout force. The term "stick" as used herein denotes the tendency of two surfaces in stationary contact to develop a degree of adherence to each other. The term "inertia" is conventionally defined as the indisposition to motion which must be overcome to set a mass in motion. In the context of the present invention, inertia is understood to denote that component of the breakout force which does not involve adherence.
Breakout force, in particular the degree of stick, varies according to the composition of the surfaces. In general, materials having elasticity show greater stick than non-elastic materials, particularly when the surfaces are of dissimilar composition. The length of time that surfaces have been in stationary contact with each other also influences breakout force. In the syringe art, the term "parking" denotes storage time, shelf time, or the interval between filling and discharge. Parking generally increases breakout force, particularly if the syringe has been refrigerated during parking.
A conventional approach to overcoming breakout has been application of a lubricant to a surface to surface interface. Common lubricants used are hydrocarbon oils, such as mineral oils, peanut oil, vegetable oils and the like. Such products have the disadvantage of being soluble in a variety of fluids, such as vehicles commonly used to dispense medicaments. In addition, these lubricants are subject to air oxidation resulting in viscosity changes and objectionable color development. Further, they are particularly likely to migrate from the surface to surface interface. Such lubricant migration is generally thought to be responsible for the increase in breakout force with time in parking.
Silicone oils are also commonly used as lubricants. They are poor solvents and are not subject to oxidation, but migration and stick do occur, and high breakout forces are a problem.
Polytetrafluoroethylene surfaces provide some reduction in breakout forces, but this material is very expensive, and the approach has not been totally effective. Thus there is a need for a better method to overcome high breakout forces whereby smooth transition of two surfaces from stationary contact into sliding contact can be achieved.
Formation of an ionizing plasma by electromagnetic activation of a gas by either glow discharge or corona discharge and use of such a plasma for modification of a surface, in particular for deposition of a material onto a polymeric surface, are well known. Exemplary of such teachings are U.S. Pat. No. 4,364,970 to Imada et al. and U.S. Pat. No. 4,072,769 to Lidel.